The engine fuel throttle of a marine engine and the associated speed of the boat are typically controlled by a throttle lever at the helm of the boat. The helm throttle lever typically manipulates the engine throttle assembly through cabling, either mechanically or electromechanically.
The engine controls can also include a clutch control lever that controls the clutch assembly in the marine transmission. The clutch assembly includes clutch plates which are forced together by high fluid pressure to transmit engine power through the transmission to the propeller drive train.
In some boats, the engine controls further include a trolling valve, that relieves varying levels of the fluid pressure that releases and compresses the clutch plates and allows slippage in the power transmission through the marine transmission to the propeller drive train. By manipulating the trolling valve, the idle speed of the boat in water can be adjusted from a normal idle speed of four to seven knots down to perhaps one knot to enhance fishing conditions and the like.
Use of the trolling valve, however, presents a significant danger of marine transmission clutch plate assembly damage. The typically high oil pressure and associated oil circulation rate inside the marine transmission during normal operation transfers a significant amount of heat generated by the marine transmission clutch plate assembly through a heat exchanger that is cooled by raw water. However, when the pressure is reduced using a trolling valve, the flow rate is reduced, and the marine transmission clutch plates can overheat as a result of inadequate heat dissipation when the engine is revved above factory limits during use of the trolling valve.